If you are scoring between 40-42 then i do not think that you should go for the GMAT CLUB TESTS
, which are designed for those that are getting 45-47 and are aiming for a 50 in quant. If you want to boost your score by 3-5 points then you should consider more on the 650 levels which you are most likely getting wrong. I would suggest going over the PS and DS section on this forum, and use the filters to sort out the 650-700 level questions to get the most practice (literally hundreds on this forum). Attempting questions that are designed to simulate the degree of difficulty that someone scoring 90+% in quant is likely to encounter is not going to be very beneficial for you. You may become very good at them after practice and review but you are less likely to come across that level of difficulty unless you breeze through the easy - middle difficulty questions first, Therefore try to get those right and move closer to 45-47 before going for gmatclub and pushing towards 50.
I am at a similar position. In diagnostics i am getting between 40-45 and need to push to 47+.
"When you want to succeed as bad as you want to breathe, then you’ll be successful.” - Eric Thomas